PDHonline Course S247 (2 PDH) Fastener Facts Instructor: Semih Genculu, P.E. 2012 PDH Online | PDH Center 5272 Meadow Estates Drive Fairfax, VA 22030-6658 Phone & Fax: 703-988-0088 www.PDHonline.org www.PDHcenter.com An Approved Continuing Education Provider www.PDHcenter.com PDH Course S247 www.PDHonline.org Course Content Although selection of bolts, nuts and washers may appear as a difficult task, in reality it is quite simple as long as factors such as operating temperature, service environment, corrosion, vibration, initial clamping load (torque) and cyclic loading (fatigue) are carefully considered. If the main function for the fastener is strength, then steel is probably the most appropriate type to use. If the service environment is corrosive, then either steel with a protective coating or stainless steel or a nonferrous alloy should be considered. If magnetic permeability is important then an austenitic stainless steel, aluminum or copper alloy should be used. If high electrical conductivity is needed, aluminum or copper fasteners can be used. For weight saving situations aluminum is the main solution. If high strength is coupled with weight consideration, as in aerospace applications, then titanium may have to be selected. For high and low temperature service stainless steels or superalloys (with high alloying additions of nickel, molybdenum, cobalt, vanadium etc.) should be utilized. Fastener Materials Bolts can be made from many different materials but most are carbon, alloy or stainless steel. Titanium and nickel alloy bolts are also used in aerospace applications. Carbon steel is the most common fastener material. Steels are usually zinc plated (galvanized) to resist corrosion. Typically the tensile strength is around 60,000 psi (pounds/square inch) for low carbon steels. Medium carbon, heat treated fasteners can achieve 120,000 psi and low-alloy steels 150,000 psi. Some higher alloy steels can reach much higher strength levels (up to 300,000 psi) through heat treatment although in most engineering applications (other than aerospace) there is seldom a need to consider any fastener with tensile strengths over 180,000 psi. With steels the poor corrosion resistance necessitates some type of coating in most applications. Cadmium, although being phased out in recent years due to environmental concerns, provides the best corrosion protection. There is also a family of steels that provide relatively good atmospheric corrosion resistance without any coatings. These are known as weathering steels and are widely used in exposed structures such as bridges, buildings and transmission towers. These low alloy steels have significant copper content, which helps build a stable protective oxide surface film when exposed to the atmosphere. Stainless steel (corrosion resistant-CRES) bolts may come in various types ranging in ultimate strength of 70-220,000 psi. Three basic types of stainless steels (austenitic, ferritic and martensitic) have distinctly different properties. Austenitic (e.g. 303, 304, 316, 321) are non heat-treatable but their properties can be improved through cold working and strain hardening techniques. In general, the solution-annealed versions possess tensile strengths in the 75,000 psi range, cold worked ones may reach 90,000 psi and strain hardened ones my go over 100,000 psi dependent on their size. The ferritic grades, such as 430, do not respond to heat treatment and have tensile strengths of about 70,000 psi. Martensitic grades (i.e. 410, 416 and 431) are heat treatable and can reach 180,000 psi. Major advantage of stainless steel fastener use is that they usually require no additional coating for corrosion protection and have a much wider service temperature range. In the nonferrous arena, aluminum is the most common fastener material. Aluminum alloys have reasonable strength, a high strength-to-weight ratio, good corrosion resistance in most environments, excellent thermal and electrical conductivity, and perform well at low temperatures. Tensile strength may range anywhere from 13,000 psi (with pure aluminum) to above 60,000 psi with the 2XXX or 7XXX series alloys (e.g. 2024 or 7075). None of the copper alloys (brasses and bronzes) respond to heat treatment and therefore the strength increase can only be achieved through cold working. However, since many copper alloys, following forming, must be stress relieved to eliminate embrittlement, fastener strengths usually are consistent with base metal levels. Typical strength levels range from 50,000 psi in certain brass alloys to around 100,000 psi for some aluminum bronzes. ©2010 Semih Genculu Page 2 of 36 www.PDHcenter.com PDH Course S247 www.PDHonline.org Nickel base alloy have excellent strength properties combined with their superior corrosion resistance, toughness, better performance at high and low temperature extremes, which gives them a serious advantage over other alloys. However, their high cost can be a limiting factor. The most commonly used ones among this alloy group is the nickel-copper group (e.g. Monel) with a tensile strength of 80,000 psi and the heat treatable nickel-copper-aluminum, which can achieve close to 130,000 psi tensile strength. Titanium, because of its superior strength-to-weight ratio, is very popular in aerospace, missile and some chemical processing plant applications where the high cost is justified. Bolts made from Ti-4Mn- 4Al and Ti-6Al-4V have tensile strengths of 150,000 psi and other titanium alloys that can reach close to 200,000 psi are available. Plastic fasteners, which are widely used in electronic and automotive applications, are the lowest strength materials with the widely used nylon fastener having a tensile strength of around 10,000 psi. Some limitations that need to be kept in mind during fastener selection are: The plating material is usually the factor defining the maximum service temperature Carbon and alloy steel bolts become brittle at low temperatures (i.e. below 30 or 40 °F) Hydrogen embrittlement becomes a problem with high strength steel bolts especially if they are plated Heat treatable (400 series) stainless steel bolts have reduced corrosion resistance Coupling of dissimilar metals can lead to galvanic corrosion. Platings and Coatings Service environment is a significant consideration when selecting fasteners. Corrosion prevention, whether from atmospheric, galvanic, high temperature oxidation or stress corrosion attack is key to avoiding premature failures. Steel fasteners with some type of plating or coating function well in most atmospheric environments. Generally, the thicker the coating or plating, the more effective the protection. There are however, certain mechanisms that come into play with coating higher strength fasteners. Most plating processes are electrolytic and generate hydrogen. Additionally, the acid cleaning processes that are performed to prepare the surfaces to be plated are sources for hydrogen pick up by the materials. This causes hydrogen embrittlement. Therefore, for high strength fasteners, most plating processes require a baking treatment after the plating to diffuse the hydrogen out of the material. For example cadmium-plated fasteners must be baked at 375°F for 23 hours, within 2 hours of plating. Low strength materials (i.e. typically less than 32 HRC) are not susceptible to hydrogen embrittlement and do not require baking. Cadmium is extremely toxic and while it has been established that plated fasteners are not a high-risk hazard, they cannot be used in contact with food or beverages. Its environmental threat relates primarily to the plating process and subsequent handling of plating effluents. Alternative coatings are being developed to replace cadmium, however its superior corrosion protection has not yet been matched with any of the replacement coatings. Besides its high resistance to corrosion, cadmium has lubricity, which lowers the friction coefficients and narrows the range of torque-tension relationships. The service temperature limit for cadmium-plated fasteners is 450°F since cadmium melts at 600°F. Zinc is also a very common plating type. The hot-dip method is commercially known as galvanizing. Zinc can also be electrodeposited. Since zinc is a sacrificial material, it will continue to provide protection in areas where the plating may be scratched off or removed. Zinc may also be applied cold as a zinc-rich spray paint. Useful service life expectancies of zinc plated fasteners in various environments are: Zinc plated with chromate treatment having 0.15 mils (0.00015 in.) plating thickness may last up to 20 years indoors, about 4 years in rural atmosphere, 2 years in coastal locations and ©2010 Semih Genculu Page 3 of 36 www.PDHcenter.com PDH Course S247 www.PDHonline.org less than 1 year in heavily polluted industrial areas. Hot-dip galvanized fastener with an average thickness (coating weight) of 1.25 oz/sq. ft will last over 40 years in rural atmosphere, 25-30 years in coastal and about 5 years in heavily polluted industrial areas (1 oz/sq. ft = 1.7 mils). Although zinc melts at 785°F, its useful service temperature is 250°F since its corrosion inhibiting qualities degrade above 140°F. Zinc plated fasteners require more tightening torque to develop equivalent preloads in fasteners. Also, zinc without some supplementary protection develops a dull white corrosion product (white rust) on the surface. They are therefore usually given a chromate treatment, which is a chemical conversion process to cover the zinc surface with a hard non-porous film. This added coating effectively seals the surface and provides added corrosion protection. Chromate coatings are available in clear, iridescent or in a variety of colors. Although not as resistant, phosphate coatings are also used for corrosion protection. The parts may be submerged in a diluted solution of phosphoric acid for forming a mildly protective layer of either crystalline iron, manganese or zinc phosphate. The nature of the crystals make phosphated part’s surface readily painted by providing better adherence. They can also be dipped in oil or wax to improve their corrosion resistance.